Dispersion compensating nonlinear polarization amplifiers

ABSTRACT

A broadband nonlinear polarization amplifier includes an input port for inputting an optical signal having a wavelength λ. A distributed gain medium receives and amplifiers the optical signal through nonlinear polarization. The distributed gain medium has zero-dispersion at wavelength λ 0 . A magnitude of dispersion at λ is less than 50 ps/nm-km. One or more semiconductor lasers are operated at wavelengths λ p  for generating a pump light to pump the distributed gain medium. An output port outputs the amplified optical signal.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of and claims thebenefit of priority from the U.S. application Ser. No. 09/766,489, filedon Jan. 19, 2001; and U.S. application Ser. No. 09/765,972, filed onJan. 19, 2001. This application also claims priority from U.S.application Ser. No. 09/760,201, filed Jan. 12, 2001, and claims thebenefit of priority from U.S. application Ser. No. 09/558,300, filedApr. 25, 2000, which claims priority from U.S. application Ser. No.09/046,900, filed Mar. 24, 1998. All the above applications are fullyincorporated herein by reference.

BACKGROUND

[0002] 1. Field of the Invention

[0003] The present invention relates generally to optical amplifiersused in fiber-optics for telecommunications, cable television and otherfiber-optics applications, and more particularly to an optical fiberamplifier and method for producing an amplified broadband output from anoptical signal with dispersion compensation.

[0004] 2. Description of the Related Art

[0005] Because of the increase in data intensive applications, thedemand for bandwidth in communications has been growing tremendously. Inresponse, the installed capacity of telecommunication systems has beenincreasing by an order of magnitude every three to four years since themid 1970s. Much of this capacity increase has been supplied by opticalfibers that provide a four-order-of-magnitude bandwidth enhancement overtwisted-pair copper wires.

[0006] To exploit the bandwidth of optical fibers, two key technologieshave been developed and used in the telecommunication industry: opticalamplifiers and wavelength-division multiplexing (WDM). Opticalamplifiers boost the signal strength and compensate for inherent fiberloss and other splitting and insertion losses. WDM enables differentwavelengths of light to carry different signals parallel over the sameoptical fiber. Although WDM is critical in that it allows utilization ofa major fraction of the fiber bandwidth, it would not be cost-effectivewithout optical amplifiers. In particular, a broadband optical amplifierthat permits simultaneous amplification of many WDM channels is a keyenabler for utilizing the full fiber bandwidth.

[0007] Silica-based optical fiber has its lowest loss window around 1550nm with approximately 25 THz of bandwidth between 1430 and 1620 nm. Forexample, FIG. 1 illustrates the loss profile of a 50 km optical fiber.In this wavelength region, erbium-doped fiber amplifiers (EDFAs) arewidely used. However, as indicated in FIG. 2, the absorption band of aEDFA nearly overlaps its the emission band. For wavelengths shorter thanabout 1525 nm, erbium-atoms in typical glasses will absorb more thanamplify. To broaden the gain spectra of EDFAs, various dopings have beenadded. For example, as shown in FIG. 3a, codoping of the silica corewith aluminum or phosphorus broadens the emission spectrum considerably.Nevertheless, as depicted in FIG. 3b, the absorption peak for thevarious glasses is still around 1530 nm.

[0008] Hence, broadening the bandwidth of EDFAs to accommodate a largernumber of WDM channels has become a subject of intense research. As anexample of the state-of-the-art, a two-band architecture for anultra-wideband EDFA with a record optical bandwidth of 80 nm has beendemonstrated. To obtain a low noise figure and high output power, thetwo bands share a common first gain section and have distinct secondgain sections. The 80 nm bandwidth comes from one amplifier (so-calledconventional band or C-band) from 1525.6 to 1562.5 nm and anotheramplifier (so-called long band or L-band) from 1569.4 to 1612.8 nm. Asother examples, a 54 nm gain bandwidth achieved with two EDFAs in aparallel configuration, i.e., one optimized for 1530-1560 nm and theother optimized for 1576-1600 nm, and a 52 nm EDFA that used two-stageEDFAs with an intermediate equalizer have been demonstrated.

[0009] These recent developments illustrate several points in the searchfor broader bandwidth amplifiers for the low-loss window in opticalfibers. First, bandwidth in excess of 40-50 nm require the use ofparallel combination of amplifiers even with EDFAs. Second, the 80 nmbandwidth may be very close to the theoretical maximum. The shortwavelength side at about 1525 nm is limited by the inherent absorptionin erbium, and long wavelength side is limited by bend-induced losses instandard fibers at above 1620 nm. Therefore, even with these recentadvances, half of the bandwidth of the low-loss window, i.e., 1430-1530nm, remains without an optical amplifier.

[0010] There is a need for nonlinear polarization amplifiers thatprovide a low noise figure amplification for operation near the zerodispersion wavelength of fibers. There is a further need for a broadbandfiber transmission system that includes nonlinear polarizationamplifiers which provide low noise amplification near the zerodispersion wavelength of fibers.

SUMMARY OF THE INVENTION

[0011] Accordingly, an object of the present invention is to provide abroadband nonlinear polarization amplifier.

[0012] Another object of the present invention is to provide a broadbandnonlinear polarization amplifier with a distributed gain medium.

[0013] A further object of the present invention is to provide abroadband nonlinear polarization amplifier that has a distributed gainmedium with a magnitude of dispersion that is less than 50 ps/nm-km.

[0014] Yet another object of the present invention is to provide abroadband nonlinear polarization amplifier with a transmission line thatincludes a Raman amplifier, and at least a portion of the transmissionline with a magnitude of dispersion less than 50 ps/nm-km.

[0015] Another object of the present invention is to provide anamplifier module that includes a dispersion compensating fiber with anegative sign of dispersion and an absolute magnitude of dispersion ofat least 50 ps/nm-km.

[0016] A further object of the present invention is to provide anamplifier module that has a transmission fiber and a dispersioncompensating fiber, where a difference between the relative dispersionslopes of the transmission fiber and the dispersion compensating fiberis no greater than 0.0032/nm over at least a portion of a signalwavelength range.

[0017] Yet another object of the present invention is to provide anamplifier module that includes a dispersion compensating fiber a pumpsource that produces a depolarized pump beam.

[0018] Still a further object of the present invention is to provide anoptical fiber communication system that includes a dispersioncompensating fiber with at least a portion having a negative sign ofdispersion and an absolute magnitude of dispersion of at least 50ps/nm-km.

[0019] These and other objects of the present invention are achieved ina broadband nonlinear polarization amplifier with an input port forinputting an optical signal having a wavelength λ. A distributed gainmedium receives and amplifiers the optical signal through nonlinearpolarization. The distributed gain medium has zero-dispersion atwavelength λ₀. A magnitude of dispersion at λ is less than 50 ps/nm-km.One or more semiconductor lasers are operated at wavelengths λ_(p) forgenerating a pump light to pump the distributed gain medium. An outputport outputs the amplified optical signal.

[0020] In another embodiment of the present invention, a broadband fibertransmission system includes a transmission line having at least onezero dispersion wavelength λ_(o) and transmitting an optical signal ofλ. The transmission line includes a Raman amplifier that amplifies theoptical signal through Raman gain. At least a portion of thetransmission line has a magnitude of dispersion at λ less than 50ps/nm-km. One or more semiconductor lasers are operated at wavelengthsλ_(p) and can generate a pump light to pump the Raman amplifier. λ isclose to λ₀ and λ₀ is less than 1540 nm or greater than 1560 nm.

[0021] In another embodiment of the present invention, a broadband fibertransmission system includes a transmission line having at least onezero dispersion wavelength λ_(o), and transmitting an optical signal ofλ. The transmission line includes a Raman amplifier and a discreteoptical amplifier that amplify the optical signal. At least a portion oftransmission line has a magnitude of dispersion at λ less than 50ps/nm-km. One or more semiconductor lasers are operated at wavelengthsλ_(p) and can generate a pump light to pump the amplifiers. λ is closeto λ₀ and λ₀ is less than 1540 nm or greater than 1560 nm

[0022] In another embodiment of the present invention, an amplifiermodule includes a transmission fiber configured to transmit a signal. Adispersion compensating fiber has at least a portion with a negativesign of dispersion and an absolute magnitude of dispersion of at least50 ps/nm-km. A first intermediate fiber couples the dispersioncompensating fiber with the transmission fiber. The first intermediatefiber has a mode field diameter that is less than a mode field diameterof the transmission fiber and greater than a mode field diameter of thedispersion compensating fiber. At least a first pump source is coupledto the transmission fiber. The first pump source produces a first pumpbeam that creates Raman gain in the dispersion compensating fiber.

[0023] In another embodiment of the present invention, an amplifiermodule includes a transmission fiber has a relative dispersion slope andis configured to transmit a signal. A dispersion compensating fiber hasa relative dispersion slope and is coupled to the transmission fiber. Adifference between the relative dispersion slopes of the transmissionfiber and the dispersion compensating fiber is no greater than 0.0032/nmover at least a portion of a signal wavelength range. At least a firstpump source is coupled to the transmission fiber and produces a firstpump beam that creates Raman gain in the dispersion compensating fiber.

[0024] In another embodiment of the present invention, an amplifiermodule includes a transmission fiber configured to transmit a signal. Adispersion compensating fiber is coupled to the transmission fiber. Atleast a first pump source is coupled to the transmission fiber. Thefirst pump source produces a depolarized first pump beam that createsRaman gain in the dispersion compensating fiber.

[0025] In another embodiment of the present invention, an optical fibercommunication system includes a transmitter, a receiver and atransmission fiber coupled to the transmitter and the receiver. Thetransmission fiber exhibits chromatic dispersion at a system wavelength.A dispersion compensating fiber is also included. At least a portion ofthe dispersion compensating fiber has a negative sign of dispersion andan absolute magnitude of dispersion of at least 50 ps/nm-km. A firstintermediate fiber couples the dispersion compensating fiber with thetransmission fiber. The first intermediate fiber has a mode fielddiameter that is less than a mode field diameter of the transmissionfiber and greater than a mode field diameter of the dispersioncompensating fiber. At least a first pump source is coupled to thetransmission fiber and produces a first pump beam that creates Ramangain in the dispersion compensating fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] These and other objects, features and elements of the presentinvention will be better understood from the following detaileddescription of preferred embodiments of the invention in which:

[0027]FIG. 1 depicts the loss profile of a 50 km fiber and the gain bandof a typical EDFA.

[0028]FIG. 2 depicts absorption and gain spectra of an EDFA.

[0029]FIG. 3a depicts emission spectra of four EDFAs with different corecompositions.

[0030]FIG. 3b depicts absorption cross-section of erbium-doped glass ofdifferent compositions.

[0031]FIG. 4 depicts a measured Raman-gain spectrum for fused silica ata pump wavelength of 1000 nm.

[0032]FIG. 5 plots power gain coefficient 2 g versus phase vectormismatch Δk for parametric amplification.

[0033]FIG. 6 demonstrates basic concepts of the NLPA of the invention.

[0034]FIG. 7 illustrates the spectral broadening and gain expected fromPA for a pump power of 1W and different separations between the pump andzero-dispersion wavelength.

[0035]FIG. 8 illustrates the spectral broadening and gain expected fromPA for a pump and zero-dispersion wavelength separation of 1 nm and forvarying pump powers.

[0036]FIG. 9a, 9 b and 9 c are schematic illustrations of embodiments ofan NLPA using an open-loop configuration.

[0037]FIG. 10 is a schematic illustration of a second embodiment of anNLPA using a Sagnac Raman cavity that is pumped at 1240 nm.

[0038]FIG. 11 is a schematic illustration of a third embodiment of anNLPA using a Sagnac Raman cavity that is pumped at 1117 nm.

[0039]FIG. 12 is a schematic illustration of a first embodiment of aparallel optical amplification apparatus having two stages of NLPAs.

[0040]FIG. 13 is a schematic illustration of a second embodiment of aparallel optical amplification apparatus that is a combination of anEDFA and an NLPA.

[0041]FIG. 14 is a schematic diagram of one embodiment of a dual stageamplifier.

[0042]FIG. 15 is a graph of gain versus wavelength for an S band dualstage amplifier, such as for the embodiment of FIG. 14.

[0043]FIG. 16 is a graph of noise figure versus wavelength for an S banddual stage amplifier, such as for the embodiment of FIG. 14.

[0044]FIG. 17 is a block chart of various embodiments of uses ofamplifiers.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0045] Some embodiments provide a structure for exploiting almost thefull 25 THz of bandwidth available in the low-loss window of opticalfibers from 1430 nm to 1620 nm. The broadband NLPA amplifier of someembodiments combines Raman amplification with either PA or 4WM toachieve bandwidth performance improvements that neither technology byitself has heretofore been able to deliver.

[0046] The broadband NLPA of other embodiments comprise an input portfor inputting an optical signal having a wavelength λ, a distributedgain medium for receiving the optical signal and amplifying andspectrally broadening the same therein through nonlinear polarization, apump source operated at wavelength λ_(p) for generating a pumping lightto pump the distributed gain medium, and an output port for outputtingthe amplified and spectrally broadened optical signal. The distributedgain medium can have zero-dispersion at wavelength λ₀ such thatλ≧λ₀≧λ_(p). The pumping light can cascade through the distributed gainmedium a plurality of Raman orders including an intermediate orderhaving a wavelength λ_(r) at a close proximity to the zero-dispersionwavelength λ₀ to phase match four-wave mixing (if λ_(r)<λ₀) orparametric amplification (if λ_(r)>λ₀).

[0047] A first embodiment of the NLPA uses open-loop amplification withan optical fiber gain medium. A pump source operated at 1240 mn can beused. The pump may be retro-reflected to increase the conversionefficiency. A second embodiment of the NLPA can use a Sagnac Ramancavity that is pumped at 1240 nm. Feedback in the Sagnac Raman cavitycan reduce the required pump power, and the broadband cavity designsupports much of the generated bandwidth. Another embodiment of the NLPAcan use a Sagnac Raman cavity pumped at 1117 nm for a very broadbandoperation.

[0048] Other embodiments relate to a parallel optical amplificationapparatus having a combination of optical amplifiers. In one embodiment,the parallel optical amplification apparatus comprises two parallelstages of NLPAs with one NLPA optimized for 1430 to 1480 nm and theother for 1480 to 1530 nm. In another embodiment, the full 25 THz of thelow-loss window in optical fibers can be exploited by a parallelcombination of a Raman amplifier and a rare earth doped amplifier. Inone embodiment, an NLPA can cover the low-loss window of approximately1430 nm to 1530 nm, and an EDFA can cover the low-loss window ofapproximately 1530 nm to 1620 nm.

[0049] Stimulated Raman scattering effect, PA and 4WM can be result ofthird-order nonlinearities that occur when a dielectric material such asan optical fiber is exposed to intense light. The third-order nonlineareffect can be proportional to the instantaneous light intensity.

[0050] Stimulated Raman scattering can be an important nonlinear processthat turns optical fibers into amplifiers and tunable lasers. Raman gaincan result from the interaction of intense light with optical phonons insilica fibers, and Raman effect leads to a transfer of energy from oneoptical beam (the pump) to another optical beam (the signal). The signalcan be downshifted in frequency (or upshifted in wavelength) by anamount determined by vibrational modes of silica fibers. The Raman gaincoefficient g_(r) for the silica fibers is shown in FIG. 4. Notably, theRaman gain g_(r) can extend over a large frequency range (up to 40 THz)with a broad peak centered at 13.2 THz (corresponding to a wavelength of440 cm⁻¹). This behavior over the large frequency range can be due tothe amorphous nature of the silica glass and enable the Raman effect tobe used in broadband amplifiers. The Raman gain can depend on thecomposition of the fiber core and can vary with different dopantconcentrations.

[0051] Raman amplification has some attractive features. First, Ramangain can upgrade existing fiber optic links because it is based on theinteraction of pump light with optical phonons in the existing fibers.Second, in some embodiments there is no excessive loss in the absence ofpump power—an important consideration for system reliability.

[0052] Raman cascading is the mechanism by which optical energy at thepump wavelength is transferred, through a series of nonlinearpolarizations, to an optical signal at a longer wavelength. Eachnonlinear polarization of the dielectric can produce a molecularvibrational state corresponding to a wavelength that is offset from thewavelength of the light that produced the stimulation. The nonlinearpolarization effect can be distributed throughout the dielectric,resulting in a cascading series of wavelength shifts as energy at onewavelength excites a vibrational mode that produces light at a longerwavelength. This process can cascade through numerous orders. Becausethe Raman gain profile can have a peak centered at 13.2 THz in silicafibers, one Raman order can be arranged to be separated from theprevious order by 13.2 THz.

[0053] Cascading makes stimulated Raman scattering amplifiers verydesirable. Raman amplification can be used to amplify multiplewavelengths (as in wavelength division multiplexing) or short opticalpulses because the gain spectrum can be very broad (a bandwidth ofgreater than 5 THz around the peak at 13.2 THz). Cascading can enableRaman amplification over a wide range of different wavelengths. Byvarying the pump wavelength or by using cascaded orders of Raman gain,the gain can be provided over the entire telecommunications windowbetween 1300 nm and 1600 nm.

[0054] Parametric amplification and 4 wave mixing (PA/4WM) involve twopump (P) photons that create Stokes (S) and anti-Stokes (A) photons.Both PA/4WM and Raman amplification arise from the third ordersusceptibility χ⁽³⁾ in optical fibers. More specifically, the real partof χ⁽³⁾, the so-called nonlinear index of refraction n₂, is responsiblefor PA/4WM, while the imaginary part of χ⁽³⁾ associated with molecularvibrations corresponds to the Raman gain effect. In silica fibers ofsome embodiments, about ⅘ths of the n₂ is an electronic, instantaneousnonlinearity caused by ultraviolet resonances, while about ⅕th of n₂arises from Raman-active vibrations, e.g., optical phonons. Theimaginary part of this latter contribution corresponds to the Raman gainspectrum of FIG. 4.

[0055] Whereas Raman amplification is attractive for providing opticalgain, PA/4WM can offer an efficient method to broaden the bandwidth ofthe optical gain. PA/4WM can have a much smaller frequency separationbetween pump and signal than Raman amplification, and the frequencydifference may depend on the pump intensity. As in Raman amplification,one advantage of PA/4WM gain is that it can be present in every fiber.However, unlike the Raman effect, both PA and 4WM can requirephase-matching. 4WM can be inefficient in long fibers due to therequirement for phase-matching. However, PA can act asself-phase-matched because the nonlinear index of refraction is used tophase match the pump and sidebands. This can be true in embodimentsoperating near the zero-dispersion wavelength in fibers. When 4WM and PAoccur near the zero-dispersion wavelength of a single-mode fiber,phase-matching can become automatic in the fiber. In 4WM, sidebands canbe generated without gain when the pump wavelength falls in the normaldispersion regime (where the pumping wavelength is shorter than thezero-dispersion wavelength). PA is 4-photon amplification in which thenonlinear index of refraction is used to phase match the pump andsidebands. For PA the pump wavelength can lie in the anomalous groupvelocity regime (i.e., where the pumping wavelength is longer than thezero-dispersion wavelength) and proper phase matching can require thatpump and signal be co-propagating in some embodiments.

[0056] To illustrate the PA/4WM gain, the gain coefficient can bederived as: $\begin{matrix}{g = \sqrt{\left( {\gamma \quad P} \right)^{2} - \left\lbrack {\left( \frac{\Delta \quad \kappa}{2} \right) + {\gamma \quad P}} \right\rbrack^{2}}} & 1\end{matrix}$

[0057] The first term under the square root sign corresponds to thethird order nonlinearity that couples the pump photons to the sidebands.The second term corresponds to the phase mismatch between the waves andit consists of two parts: one due to the wave-vector mismatch at thedifferent wavelengths and the other due to the increase in nonlinearindex induced by the pump. The nonlinearity parameter can be defined as$\begin{matrix}{\gamma = {{\frac{\omega}{c}\frac{n_{2}}{A_{eff}}} = {\frac{2\quad \pi}{\lambda}\frac{n_{2}}{A_{eff}}}}} & 2\end{matrix}$

[0058] Some embodiments operate near the zero-dispersion wavelength λ₀,and the propagation constant can be expanded as: $\begin{matrix}{\left. {{\Delta \quad \kappa} = {{\frac{\lambda^{2}}{2\pi \quad c}\left\lbrack \frac{D}{\lambda} \right.}_{\lambda_{0}}\left( {\lambda_{p} - \lambda_{0}} \right)}} \right\rbrack \Omega^{2}} & 3\end{matrix}$

[0059] where

Ω=ω_(p)−ω_(s)=ω_(a)−ω_(p).  4

[0060] The pump wavelength can falls in the normal dispersion regime forsome embodiments, and D<0,∂D/∂λ>0, (λ_(p)−λ₀)<0, so that Δk>0. In thiscase, g can be imaginary, and there may be no gain during the sidebandgeneration process. This can correspond to the case of 4WM. Someembodiments operate in the anomalous group velocity dispersion regime,and D>0,∂D/∂λ>0, (λ_(p)−λ_(O))>0, so that Δk<0. This can be the regimeof PA, and the nonlinearity helps to reduce the phase mismatch (i.e.,the two parts in the second term in Equation (1) are of opposite sign).There can be gain for PA, and the gain can be tunable with the pumppower. For example, the power gain coefficient 2 g is plottedschematically in FIG. 5 for operation in the anomalous group velocityregime. The peak gain (g_(peak)=γP) can occur at Δk_(peak)=−2γP. Therange over which the gain exists can be given by 0>Δk>−4γP in someembodiments. Thus, the peak gain can be proportional to the pump power,and the Δk range can be determined by the pump power.

[0061] Consequently, from Equation (2) the bandwidth can be increased byincreasing the pump power, increasing the nonlinear coefficient n₂ ordecreasing the effective area A_(eff). In other embodiments, for a givenrequired frequency range over which gain is required, the pumprequirements can be reduced by increasing the effective nonlinearity(n₂/A_(eff)).

[0062] Several embodiments lead to broadband gain for cascaded Ramanamplification by arranging at least one intermediate Raman cascade orderat close proximity to the zero-dispersion wavelength λ₀ (e.g., within ±5nm of λ₀ for some embodiments; within ±2 nm for other embodiments).Either 4WM (if λ_(r)<λ₀) or PA (if λ_(r)>λ₀) can lead to spectralbroadening of that particular Raman order. In subsequent Raman ordersthe bandwidth can grow even further. In other embodiments, the cascadeRaman wavelength λ_(r) lies to the long wavelength side of λ₀ (i.e., inthe anomalous dispersion regime), so that parametric amplification canoccur.

[0063] An embodiment of the broadband NLPA is illustrated in FIG. 6.Starting from the pump wavelength λ_(p), cascaded Raman amplificationcan be used in the first few stages. The pump can be more than one Ramanshift or 13.2 THz away from the zero-dispersion wavelength. To keephigher efficiency in these initial steps, some embodiments can use anarrow band cavity design, such as designs based on gratings orwavelength selective couplers.

[0064] Some embodiments broaden the gain bandwidth by positioning one ofthe intermediate Raman cascade orders at a close proximity to thezero-dispersion wavelength λ₀. By operating close to λ₀, it can almostautomatically phase-match either 4WM or PA. In the subsequent cascadedRaman orders, the gain bandwidth may continue to broaden. This occursbecause the effective gain bandwidth of Raman is the convolution of thebandwidth of the pump (in this case, the previous Raman cascade order)with the Raman gain curve. In some embodiments with Raman amplification,the gain spectrum follows the pump spectrum. As the pump wavelengthchanges, the Raman gain can change as well, separated by the distance ofoptical phonon energy which in silica fibers is an approximately 13.2THz down-shift in frequency.

[0065] If the fiber is conventional so-called standard fiber, thenzero-dispersion wavelength λ₀ can be about 1310 nm. Fordispersion-shifted fiber, the zero-dispersion wavelength λ₀ can shift tolonger wavelengths by adding waveguide dispersion. In other embodiments,a dispersion-flattened fiber can be used for low dispersion values overone or more of the Raman cascade orders. In some embodiments withdispersion-flattened fiber, the dispersion slope can be small, so thegain bandwidth can be even larger (c.f. Equations (1) and (3)).

[0066] The Raman gain spectrum can follow the pump spectrum, such aswhen there is nothing in the Raman cavity to restrict the bandwidth ofthe subsequent orders. For these higher cascade order Raman laserschemes, some embodiments use gratings or wavelength selective couplers.Other embodiments with the broadband cavity design of the Sagnac Ramanamplifier and laser can have increased bandwidth with a tailored pumpspectrum. A single-pass fiber design can constitute the broadestbandwidth design. A broadband cavity such as the Sagnac Raman cavity canhave the feedback used to lower the threshold and the required pumppower. Broadening the bandwidth can lead to a drop in efficiency, so thepump powers can be higher for the broadband cavity designs.

[0067] Cascaded Raman amplification can reach the 1430-1530 nm range ofthe low-loss window. Pumping can occur with a commercially availablecladding-pumped fiber laser, which operates around 1060 to 1140 nm. Thevarious Raman orders, each separated by 13.2 Thz from the previousorder, are set forth in Table 1.

[0068] Table 1. Various Raman orders when pumping between 1060 and 1140nm (separation of 13.2 THz between orders) Wavelength (nm) Δλ Wavelength(nm) Δλ 1060.00  51.86 1110.00  57.00 1111.86  57.19 1167.00  63.171169.05  63.39 1230.16  70.40 1232.44  70.66 1300.56  78.94 1303.11 79.26 1379.50  89.14 1382.37  89.53 1468.64 101.46 1471.90 101.931570.10 116.52 1573.82 117.09 1686.62 135.20 Wavelength (nm) ΔλWavelength (nm) Δλ 1070.00  52.86 1117.00  57.74 1122.86  58.36 1174.74 64.03 1181.22  64.76 1238.77  71.41 1245.98  72.27 1310.18  80.151318.25  81.17 1390.33  90.59 1399.42  91.82 1480.92 103.22 1491.25104.72 1584.15 118.69 1595.97 120.54 1702.84 137.92 Wavelength (nm) ΔλWavelength (nm) Δλ 1080.00  53.88 1120.00  58.05 1133.88  59.54 1178.05 64.40 1193.42  66.14 1242.46  71.85 1259.56  73.90 1314.31  80.671333.47  83.11 1394.98  91.22 1416.58  94.16 1486.20 103.99 1510.74107.57 1590.19 119.63 1618.32 124.07 1709.82 139.10 Wavelength (nm) ΔλWavelength (nm) Δλ 1090.00  54.91 1130.00  59.12 1144.91  60.74 1189.12 65.65 1205.65  67.54 1254.77  73.32 1273.19  75.56 1328.10  82.431348.74  85.09 1410.53  93.33 1433.83  96.55 1503.86 106.56 1530.38110.49 1610.42 122.81 1640.87 127.69 1733.24 143.09 Wavelength (nm) ΔλWavelength (nm) Δλ 1100.00  55.95 1140.00  60.20 1155.95  61.94 1200.20 66.92 1217.89  68.96 1267.12  74.82 1286.85  77.24 1341.93  84.211364.09  87.10 1426.14  95.48 1451.19  98.98 1521.62 109.18 1550.17113.47 1630.81 126.07 1663.64 131.40 1756.87 147.19

[0069] To obtain gain between 1430 nm and 1520 nm, the pump can beoperated between 1090 nm and 1140 nm, and five cascaded Raman orders canbe used to reach the desired wavelength. To make use of the broadeningfrom PA or 4WM, a pumping scheme can be selected in the middle of thisrange, i.e., starting with a pump wavelength of 1117 nm. Then, thevarious Raman orders land at approximately 1175 nm, 1240 nm, 1310 nm,1390 nm and finally 1480 nm. In particular, the third Raman frequency(1310 nm) passes through the zero-dispersion point of a standard fiber,and the next order (1390 nm) can be close if the fiber is dispersionshifted. A broadband gain can be expected for wavelengths in the1430-1530 nm range centered around 1480 nm by using a fiber with astandard dispersion and a pump wavelength of 1117 nm, 1175 mn or 1240nm.

[0070] Broadening can be expected from PA. A standard fiber can be usedand the pump wavelength can start at 1117 nm. The calculations useEquations (1-4) with the following typical parameters for high-Ramancross-section fiber in some embodiments: λ₀=1310 nm, γ=9.9W⁻¹km⁻¹, and adispersion slope of 0.05 ps/nm-km. In FIG. 7, the gain coefficient forPA is plotted versus wavelength at a pump power of 1W and wavelengthseparations (λ_(r)-λ₀) of 0.5, 1, 2 and 5 nm. For a wavelengthseparation of 2 nm, the PA peak gain occurs at ±10 nm, so the spectralbroadening is over 20 nm. The closer the pump wavelength approaches thezero-dispersion wavelength, the wider the gain bandwidth can be. Inaddition, FIG. 8 plots the gain versus wavelength for a separation of(λ_(r)-λ₀)=1 nm and pump powers of 0.7, 1, 2, and 3W. The peak gain canincrease directly proportionally to the pump power, while the bandwidthcan increase as the square root of pump power.

[0071]FIG. 9 shows a first embodiment that uses an open-loop design toproduce an amplified broadband signal for a range of wavelengths between1430 nm and 1530 nm. The open-loop design is a nonlinear polarizationamplifier, and may have a high pump power requirement. In the NLPAamplifier 20 as illustrated in FIG. 9, an optical signal having awavelength between 1430 nm and 1530 nm is input from an input port 25 toan optical fiber 30. The optical fiber 30 is pumped by a pumping lightgenerated by a pumping laser 35 operated at a wavelength of about 1240nm. The optical signal is amplified and spectrally broadened in thefiber by nonlinear polarization, and output through an output port 40.The configuration is so arranged that the optical signal can have awavelength greater than the zero-dispersion wavelength of the fiber,which in turn is greater than the pumping wavelength of 1240 nm.

[0072] In this open-loop configuration, the fiber can have a cut-offwavelength below 1240 nm to be single-mode (spatial) over allwavelengths of the Raman cascade. Three choices of the fiber embodimentscan be used in some embodiments. First, a standard dispersion fiber witha zero-dispersion wavelength at about 1310 nm. Second, two fibersspliced together with one fiber having a zero-dispersion wavelength atabout 1310 nm (first cascade) and the other at 1390 nm (second cascade).Third, a dispersion-flattened fiber with low-dispersion at least between1310 nm and 1390 nm. The reduced dispersion slope of such adispersion-flattened fiber increases significantly the bandwidth for PAor 4WM.

[0073] Exemplary 1240 nm pump lasers include: (a) an 1117 nmcladding-pumped fiber laser followed by a coupler-based or grating-basedRaman oscillator cavity (with gratings for 1117 nm, 1175 nm and 1240nm); (b) an optically-pumped semiconductor laser; or (c) achromium-doped forsterite laser. At one end of the fiber, a 1240 nmretro-reflector 45 can be placed to increase pumping conversionefficiency. The retro-reflector can be a dichroic mirror or a 1240 nmgrating. The input and output ports can be WDM couplers, and isolatorscan be used at the input and output ports to prevent lasing due tospurious feedback. A counter-propagating geometry can average out noisefluctuations in this open-loop configuration. A co-propagating geometrycan be used.

[0074] To reduce the pump power requirements, a broadband cavity such asthe Sagnac Raman cavity can be used in some embodiments. FIG. 10illustrates an embodiment of the NLPA that uses a Sagnac Raman cavitydesign with a 1240 nm pump. Referring to FIG. 10, the Sagnac Ramancavity of the NLPA 60 can be formed by a broadband mirror 70 and a loopmirror comprising a Raman gain fiber 65 and an optical coupler 90connected thereto. An optical signal can have a wavelength between 1430nm to 1530 nm input through an input port 75 to the Raman gain fiber 65.A pumping laser 80 can operate at a wavelength 1240 nm and can generatea pumping light that pumps the fiber 65 through a coupler 85. Theoptical signal can be amplified and spectrally broadened in the fiber bynonlinear polarization, and output through an output port 95. Theconfiguration can be arranged so that the optical signal has awavelength greater than the zero-dispersion wavelength of the fiber,which in turn can be greater than the pumping wavelength of 1240 nm.

[0075] The Raman gain fiber can have the same characteristics asdescribed above for the open-loop design. Similarly, the pumping lasersused in the first embodiment can be used in this second embodiment. Thebroadband NLPA may further include a polarization controller 100 in theSagnac Raman cavity for controlling polarization state. In otherembodiments, if the fiber is polarization maintained, the polarizationcontroller can be unnecessary. The optical coupler 90 is nominally 50:50at least for the optical signal having a wavelength between about 1240nm and 1430 nm. The coupler 85 can be a WDM coupler that can transmit atleast at a wavelength between about 1300 nm and 1430 nm. The input portand output port each comprises a WDM coupler which can transmit at leastat a wavelength between about 1240 nm and 1425 nm. One embodiment of theSagnac Raman cavity has a passive noise dampening property that leads toquieter cascading of various Raman orders.

[0076] In various embodiments, a Sagnac Raman cavity can be used for allfive Raman cascade orders between 1117 nm and the low-loss window. FIG.11 illustrates a third embodiment of a five-order Sagnac Raman amplifierfor NLPA operation. A cladding-pumped fiber laser operating around 1117nm can be used as a pumping laser 120. Different fiber combinationsembodiment can be used. The fibers can have a cut-off wavelength below1117 nm to accommodate single-mode operation for the pump. An opticalcoupler 130 can be nominally 50:50 at least for the optical signalhaving the wavelength between about 1117 nm and 1430 nm. A coupler 125can be a WDM coupler that can transmit at least at wavelengths betweenabout 1165 nm and 1430 nm. Moreover, the input and output ports eachcomprises a WDM coupler which can transmit at least at wavelengthsbetween about 1117 nm and 1425 nm. Although the wavelength range of thevarious components increases, this configuration can lead to an evenbroader gain band since the pump bandwidth is allowed to increase evenduring the first two cascades between 1117 nm and 1240 nm for someembodiments. Also, the noise dampening property of the Sagnac cavity canbe used over all five Raman orders for some embodiments.

[0077] Some embodiments include an NLPA. An optical signal having awavelength λ is input through an input port into a distributed gainmedium having zero-dispersion at a wavelength λ₀, such as an opticalfiber, which can be pumped by a pumping light from a pump sourceoperated at a wavelength λ_(p), wherein λ≧λ₀≧λ_(p). The pumping lightcan cascade through the distributed gain medium a plurality of Ramanorders including an intermediate order having a wavelength λ_(r) at aclose proximity to the zero-dispersion wavelength λ₀ to phase matchfour-wave mixing (if λ_(r)<λ₀) or parametric amplification (ifλ_(r)>λ₀). The amplified and spectrally broadened optical signal isoutput through an output port.

[0078] The above embodiments demonstrate that a single NLPA canaccommodate the full bandwidth of the low-loss window. Moreover, thefull bandwidth of the low-loss window may be reached by using a paralleloptical amplification apparatus having a combination of two or moreRaman amplifiers and rare earth doped amplifiers. In some embodiments,the NLPAs and EDFAs are used.

[0079]FIG. 12 shows a first embodiment of the parallel opticalamplification apparatus using a combination of two NLPAs for a range ofwavelengths between 1430 nm and 1530 nm. Referring to FIG. 12, a divider170 divides an optical signal having a wavelength between 1430 nm to1530 nm at a predetermined wavelength, such as 1480 nm, into a firstbeam having a wavelength less than the predetermined wavelength and asecond beam having a wavelength greater than the predeterminedwavelength in some embodiments. The first beam is input into a firstNLPA 180 for amplification and spectral broadening therein. The secondbeam is input into a second NLPA 190 for amplification and spectralbroadening therein. Outputs from the first and second NLPAs can becombined by a combiner 200 to produce an amplified and spectrallybroadened optical signal. The input port 170 and output port 200 can bepreferably WDM couplers in some embodiments.

[0080] In other embodiments the first NLPA 180 can be optimized for1430-1480 nm and centered at 1455 nm, while the second NLPA can beoptimized for 1480-1530 nm and centered at 1505 nm. From Table 1, thesetwo windows can be achieved in a five-order cascade by starting with apump wavelength of about 1100 nm for the short-wavelength side and apump wavelength of about 1130 nm for the long-wavelength side. For theshort-wavelength side, the fiber can have a zero-dispersion around 1365nm, while for the long-wavelength side, the fiber zero-dispersion can bearound 1328 nm or 1410 nm.

[0081] The narrower-bandwidth for each NLPA can lead to an increasedefficiency for each amplifier in some embodiments. Furthermore, thecomponents may be more easily manufactured, since the wavelength windowis not as large. The multiple amplifiers in some embodiments may allowfor gradual upgrades of systems, adding bandwidth to the EDFA window asneeded.

[0082] A spectrum of 1430-1620 nm in the low-loss window can beamplified and spectrally broadened by using a parallel opticalamplification apparatus comprising Raman amplifiers and rare earth dopedamplifiers. FIG. 13 describes a second embodiment of the paralleloptical amplification apparatus. The amplification apparatus comprises abroadband NLPA 240 and a EDFA 250. A divider 230 of the apparatusdivides an optical signal having a wavelength between 1430 nm and 1620nm at a predetermined wavelength, preferably at 1525 nm, into a firstbeam having a wavelength less than the predetermined wavelength and asecond beam having a wavelength greater than the predeterminedwavelength in some embodiments. The broadband NLPA 240 receives thefirst beam and produces an amplified broadband first beam. The EDFA 250receives the second beam and produces an amplified broadband secondbeam. A combiner 260 combines the amplified and spectrally broadenedfirst and second beams to produce an amplified broadband optical signal.Other embodiments can have WDM couplers for the divider 230 and thecombiner 260.

[0083] To use some embodiments with multi-wavelength WDM channels, atthe output of the amplifier, gain can be equalized. This wavelengthdependency or nonuniformity of the gain band can have little impact onsingle-channel transmission. However, it can render the amplifierunsuitable for multichannel operation through a cascade of amplifiers.As channels at different wavelengths propagate through a chain ofamplifiers, they can accumulate increasing discrepancies between them interms of gain and signal-to-noise ratio. Using gain-flattening elementscan significantly increase the usable bandwidth of a long chain ofamplifiers. For example, the NLPA can be followed by a gain flatteningelement to provide gain equalization for different channels in someembodiments. Alternately, the gain flattening element could beintroduced directly into the Sagnac interferometer loop in otherembodiments, such as in FIGS. 10 or 11.

[0084] Due to the high pump power requirements of Raman amplifiers, someembodiments include higher efficiency Raman amplifiers, where theefficiency can be defined as the ratio of signal output to pump input.In one embodiment, the efficiency can be improved to the point thatlaser diodes (LD's) can be used to directly pump the Raman amplifier. Asan exemplary benchmark, for a dual stage amplifier made fromdispersion-shifted fiber (DSF) with a gain of >15 dB and an electricalnoise figure of <6 dB, a pump power of about 1W can be required from theRaman oscillator or pump laser. This power level can require thecombined powers from about eight LD's in one embodiment. If the pumprequirements could be dropped by a factor of four or so, the pump powerscould be achieved with the combination of two LD's that are polarizationmultiplexed in another embodiment. In one embodiment, four LD's could beused to provide more than 0.5W of power, and the remaining improvementfactor could be used to reduce the gain fiber lengths.

[0085] One embodiment improves the efficiency of Raman amplifiers byincreasing the effective nonlinearity of the fiber used as the gainmedium. The effective nonlinear coefficient for the fiber can be definedas $\gamma = {\frac{2\quad \pi}{\lambda}\frac{n_{2}}{A_{eff}}}$

[0086] where n₂ is the nonlinear index of refraction and A_(eff) is theeffective area of the fiber. The Raman gain coefficient can be directlyproportional to γ. The Raman coefficient is the imaginary part of thenonlinear susceptibility while the index is proportional to the realpart of the susceptibility, and the nonlinear index and Raman gain willbe related by the so-called Kramers-Kronig relations. For a dispersionshifted fiber at 1550 nm wavelength with an n₂=2.6×10⁻¹⁶ cm²/W and anA_(eff)=50 μm², the nonlinear coefficient can be about γ=2 W⁻¹km⁻¹. Ifthis value is raised to over 3 W⁻¹km⁻¹, then the pump power or fiberlengths can be reduced in proportion to the increase in nonlinearcoefficient.

[0087] Beyond the constraint on the Raman gain coefficient, thedispersion in the amplifier can be restricted. To maintain a relativelylow level of dispersion in the vicinity of the signal wavelengths, thezero dispersion wavelength λ_(o) can be in close proximity to theoperating wavelength. For single-channel, high-bit-rate systems, oneembodiment minimizes the dispersion by placing the signal wavelengthwithin 10 mn of the λ_(o). For some embodiments of multi-wavelength WDMsystems, where the channels can interact through four-wave mixing in thevicinity of λ_(o), a dispersion-managed fiber can be used. Adispersion-managed fiber can have a locally high dispersion but apath-averaged value for dispersion close to zero by combining lengths ofplus and minus values for the dispersion around the operating band. Forthe operating wavelength band, some segments of fiber can have λ_(o) atshorter wavelengths and some segments of fiber can have λ_(o) at longerwavelengths.

[0088] By proper design of the fiber, higher nonlinearity and lowerdispersion can be achieved. For example, for operation in the S-bandaround 1520 nm, high nonlinearity fibers have been produced. The fibercore can have a modified parabolic refractive index profile with aΔ_(peak)=2%. Three exemplary fibers have zero dispersion wavelengths of1524 nm, 1533 nm and 1536 nm. Such fibers can have a dispersion slope of0.043 ps/nm²-km, and the loss at 1550 nm can be approximately 0.6 dB/km.The nonlinear coefficient can be γ=9 W⁻¹km⁻¹, or a factor of 4.5× higherthan in DSF. The enhancement can be attributed to two factors: a smallereffective area and a higher germanium content. The effective area can bereduced to about A_(eff)=16.5 μm², or about a factor of 3.3 less than inDSF. Also, the nonlinear index of refraction is about 1.35× larger thanin DSF due to the extra germanium used to increase Δ_(peak) from 1% inDSF to 2% for the high nonlinearity fiber. In addition the mode fielddiameter at 1550 nm can be measured to be 4.67 μm.

[0089] For the gain fiber used in the Raman amplifier, a figure-of-meritfor the fiber can be defined in some embodiments. A figure-of-merit thatcan be measured and indicate amplifier performance is the ratio of theRaman gain coefficient to the loss at the signal wavelength. The higherthis figure-of-merit, the better the performance of the amplifier. Thisfigure-of-merit for different fibers in some embodiments is provided inTable 1. In one embodiment the lowest figure-of-merit is found forstandard (non-dispersion-shifted) SMF-28 fiber. This fiber can have alow germanium content and a relatively large A_(eff)=86 μm². Thefigures-of-merit for the high-nonlinearity (Hi-NL) fiber can exceed theother fibers, with a value about two-fold larger than Lucent True-wavefiber in one example. Although the DCF's can have a relatively largefigure-of-merit for Raman amplification, they can have very largedispersion coefficients for S-band signals. TABLE 1 Comparison of Ramangain figure-of-merit for different fibers measured. Gain [dB/W-km] Loss[dB/km] Fiber Type @ 1500 nm @ 1500 nm Figure-of-Merit Corning SMF-282.2 0.19 11.6 Lucent True-Wave 3.3 0.21 15.7 Corning SMF-DS 4.0 0.2 20.0Corning DCF 11.75 0.445 26.4 Lucent DCF 13.72 0.5 27.6 Hi-NL 18.0 0.630.0

[0090] One embodiment with Hi-NL fiber has significant improvements interms of fiber length and pump power used in a Raman amplifier. Oneembodiment has an amplifier made out of Lucent True-Wave fiber. Thespecifications for the unit can be: low dispersion around 1520 nm, 15 dBof peak gain, electrical and optical NF under 6 dB, and multi-pathinterference (MPI) under 50 dB. A two-stage design for the Ramanamplifier can be used, as illustrated in FIG. 14. In particular, 6 km ofTrue-Wave fiber can be used in the first stage and 10-12 km of fiber canbe used in the second stage. The measured performance of the amplifiercan be: peak gain of 15.2 dB at 1516 nm, 3 dB bandwidth of 26 nm(between 1503-1529 nm), and electrical and optical noise figure under 6dB. For example, the gain versus wavelength and noise figure versuswavelength for the unit is illustrated in FIGS. 15 and 16. Thisperformance can have a pump power of about 1.0 W at 1421 nm.

[0091] In one embodiment, the True-Wave fiber in this design is replacedwith Hi-NL fiber. Reductions in fiber lengths and pump powerrequirements can be achieved. The Hi-NL fiber can meet the dispersionrequirement in some embodiments. The DCF fibers can lead to theintroduction of large amounts of dispersion. Referring to the Table 1comparison, the fiber lengths can be chosen to keep roughly the sameamount of net loss. In one embodiment, fiber lengths can be roughly 2 kmfor the first stage and 3.3-4 km for the second stage. Pump powerrequirements can be lowered by the ratio of figures-of-merit, or roughlyto 0.5W. in various embodiments, this power range can be provided by theRaman oscillator, or by polarization and wavelength multiplexing 3-4LD's together. Hi-NL fiber can reduce the size of the amplifier as wellas permit LD pumping in some embodiments.

[0092] The fiber can have single-mode operation for the pump as well asthe signal wavelengths in some embodiments. Cut-off wavelength λ_(c) ofthe fiber can be shorter than any of the pump wavelengths in someembodiments. The pump can be multi-mode, and noise can be introducedfrom the beating between modes in other embodiments.

[0093] Various embodiments have reduction of the Raman amplifier sizeand pump requirements while maintaining low net dispersion at theoperating wavelengths, and include one or more of:

[0094] (A) A Raman amplifier using a gain fiber characterized in that

[0095] nonlinear coefficient γ>3 W⁻¹km−1

[0096] zero dispersion wavelength in the range of 1300<λ_(o)<1800 nm,depending more precisely on the specifications

[0097] Loss over the operating wavelength of <2 dB/km, with a preferencefor loss <1 dB/km

[0098] (B) A Raman amplifier using a dispersion managed gain fibercharacterized in that

[0099] nonlinear coefficient γ>3 W⁻¹km⁻¹

[0100] dispersion management done using segments of fiber with zerodispersion wavelength in the range of 1300<λ_(o)<1800 nm, depending moreprecisely on the specifications. Given an operating band, certain fibersegments have λ_(o) less than the operating band and other fibersegments have λ_(o) greater than the operating band. The localdispersion can be kept high, while the path average dispersion can beclose to zero in the signal band.

[0101] Loss over the operating wavelength of certainly <2 dB/km, with apreference for loss <1 dB/km

[0102] (C) Fibers as in (A) or (B) with cut-off wavelength shorter thanany of the pump wavelengths.

[0103] (D) A Raman amplifier as described in (A) that is pumped by LD's.For two or more LD's, the power can be combined by using polarizationand wavelength multiplexing using polarization beam combiners andwavelength-division-multiplexers.

[0104] (E) A Raman amplifier as in (B) that is pumped by LD's. For twoor more LD's, the power can be combined by using polarization andwavelength multiplexing using polarization beam combiners andwavelength-division-multiplexers.

[0105] (F) At least a two-stage Raman amplifier that uses theimprovements in (A),(B),(C),(D) or (E).

[0106] (G) Other factors as above with different numerical ranges

[0107] Some embodiments include standard dispersion fiber, i.e., fiberswith zero dispersion wavelength around 1310 nm. The zero dispersionwavelength can fall in the S− or S⁺-bands in some embodiments. Forexample, this is true for so-called non-zero-dispersion-shifted fiber(NZ-DSF). In these fibers, it can be difficult to run multi-wavelengthWDM channels due to cross-talk from four-wave mixing. Four-wave-mixingcan require phase matching, and the phase matching can be easier tosatisfy in the neighborhood of the zero dispersion wavelength. Oneembodiment is a broadband fiber transmission system with non-zerodispersion fiber that has zero dispersion wavelengths less than 1540 nmor greater than 1560 nm that uses optical amplifiers to compensate forloss.

[0108] WDM can maximize capacity in any given band in some embodiments.Hybrid amplifiers can be useful in the vicinity of the zero dispersionwavelength in some embodiments. NZ-DSF fibers can have a zero dispersionwavelength either <1540 nm or >1560 nm in some embodiments. Foroperation near the zero dispersion wavelength, e.g., |λ−λ_(o)|<25 nm,the four-wave-mixing penalty can be avoided by using hybrid opticalamplifiers in one embodiment. Since the effective NF of hybridamplifiers can be lower than for discrete amplifiers, the power levelsfor the signals can be reduced to the point that four-wave-mixing can nolonger be a limitation, in another embodiment.

[0109] One embodiment of a broadband nonlinear polarization amplifiercomprises an input port, a distributed gain medium, one or moresemiconductor lasers, and an output port. The input port can input anoptical signal having a wavelength λ. The input port can be a WDMcoupler. The wavelength λ can be between 1400 nm and 1650 nm. A sign ofdispersion at the wavelength λ can be negative. The distributed gainmedium can receive the optical signal and can amplify the optical signalthrough nonlinear polarization. The distributed gain medium can havezero-dispersion at wavelength λ₀, which can be in the range of 1300 to1800 nm. The distributed gain medium can be an optical fiber, or adispersion compensating optical fiber. The distributed gain medium canbe a dispersion managed gain medium with a plurality of fiber segmentseach having a zero dispersion wavelength in the range of 1300 to 1800nm. At least a portion of gain produced by the distributed gain mediumcan be Raman gain. The distributed gain medium can have a nonlinearcoefficient greater than 2 W-1km-1, or greater than 3 W-1km-1. Thedistributed gain medium can have a cut-off wavelength shorter than thewavelengths λ_(p). The distributed gain medium can have a loss at thewavelength λ of less than 2 dB/km, or less than 1 dB/km. A magnitude ofdispersion at the wavelength λ can be less than 50 ps/nm-km, less than40 ps/nm-km, less than 30 ps/nm-km, or less than 20 ps/nm-km. One ormore semiconductor lasers can be operated at wavelengths λ_(p). One ormore semiconductor lasers can generate a pump light to pump thedistributed gain medium. The output port can output the amplifiedoptical signal. The output port can be a WDM coupler.

[0110] One embodiment of a broadband fiber transmission system comprisesa transmission line and one or more semiconductor lasers. Thetransmission line can have at least one zero dispersion wavelength λ₀.The transmission line can transmit an optical signal of a wavelength λ.The optical signal can have a wavelength λ in the range of 1400 nm to1530 nm, and/or in the range of 1530 nm to 1650 nm. A sign of dispersionat the wavelength λ can be negative. A signal wavelength at λ can besufficiently low in power to avoid at least one fiber non-linearityeffect, such as four-wave mixing and/or modulation instability. Thewavelength λ can be within, for example, 30 nm, or within 20 nm, of atleast one zero dispersion wavelength λ₀. The transmission line caninclude a Raman amplifier. The Raman amplifier can amplify the opticalsignal through Raman gain. The Raman amplifier can be a distributedRaman amplifier. The Raman amplifier can have sufficient gain tocompensate for losses in the transmission line. At least a portion ofthe transmission line can have a magnitude of dispersion at thewavelength A less than 50 ps/nm-km, less than 40 ps/nm-km, less than 30ps/nm-km, or less than 20 ps/nm-km. One or more semiconductor lasers canbe operated at wavelengths λ_(p). One or more semiconductor lasers cangenerate a pump light to pump the Raman amplifier. The wavelength λ canbe close to at least one zero dispersion wavelength λ₀. At least onezero dispersion wavelength λ₀ can be less than 1540 nm and/or greaterthan 1560 nm.

[0111] One embodiment of a broadband fiber transmission system comprisesa transmission line and one or more semiconductor lasers. Thetransmission line can have at least one zero dispersion wavelength λ₀.The transmission line can transmit an optical signal of a wavelength λ.The optical signal can have the wavelength λ in the range of 1400 nm to1530 nm, and/or in the range of 1530 nm to 1650 nm. A signal wavelengthat λ can be sufficiently low in power to avoid at least one fibernon-linearity effect, such as four-wave mixing and/or modulationinstability. The wavelength A can be within 30 nm, or within 20 nm, ofat least one zero dispersion wavelength λ₀. A sign of dispersion at thewavelength λ can be negative. The transmission line can include a Ramanamplifier and a discrete optical amplifier that amplify the opticalsignal. The Raman amplifier can be a distributed Raman amplifier. Thediscrete optical amplifier can be a rare earth doped amplifier, anerbium doped fiber amplifier, a Raman amplifier, and/or a thulium dopedfiber amplifier. The amplifiers can have sufficient gain to compensatefor losses in the transmission line. At least a portion of thetransmission line can have a magnitude of dispersion at the wavelength λless than 50 ps/nm-km, less than 40 ps/nm-km, less than 30 ps/nm-km, orless than 20 ps/nm-km. One or more semiconductor lasers can be operatedat wavelengths λ_(p). One or more semiconductor lasers can generate apump light to pump the amplifiers. The wavelength λ can be close to atleast one zero dispersion wavelength λ₀. At least one zero dispersionwavelength λ₀ can be less than 1540 nm and/or greater than 1560 nm.

[0112] An embodiment of a amplifier module comprises a transmissionfiber, a dispersion compensating fiber, a first intermediate fiber, andat least a first pump source. The transmission fiber can be configuredto transmit a signal. The signal can be, for example, in the wavelengthrange of 1400 to 1650 nm. The signal can include multiple wavelengths.The transmission fiber can be coupled to a second pump source thatproduces a second pump beam. The first and second pump beams can beseparated by at least 15 nm, or at least 20 nm, of wavelength. At leasta portion of the dispersion compensating fiber can have a negative signof dispersion and an absolute magnitude of dispersion of at least 50ps/fm-km for a majority of wavelengths in the signal, or at least aportion of the wavelengths in the signal. The dispersion compensatingfiber can include at least two dispersion compensating fibers. The firstintermediate fiber can couple the dispersion compensating fiber with thetransmission fiber. The first intermediate fiber can have a mode fielddiameter that can be less than a mode field diameter of the transmissionfiber and greater than a mode field diameter of the dispersioncompensating fiber. The first intermediate fiber can couple thedispersion compensating fiber with the transmission fiber with no morethan 1 dB of loss over at least a portion of a wavelength range of thefirst pump source. The first intermediate fiber can be integrally formedwith the transmission fiber, and/or with the dispersion compensatingfiber, and can have a varying mode field diameter. The firstintermediate fiber can be non-integrally formed with at least one of, orboth of, the transmission fiber and the dispersion compensating fiber.The first intermediate fiber can have substantially tapered geometricconfigurations. At least the first pump source can be coupled to thetransmission fiber. At least the first pump source can produce a firstpump beam that creates Raman gain in the dispersion compensating fiber.The first pump source can include at least one semiconductor diodelaser. The amplifier module provides net gain to a signal in thetransmission fiber of at least 5 dB, or at least 10 dB. The amplifiermodule can have sufficient gain to compensate for signal losses in thetransmission line. The amplifier module can further comprise awavelength selective coupler. The wavelength selective coupler can becoupled to an output of the dispersion compensating fiber. Thewavelength selective coupler can provide for counter-directionalpumping.

[0113] One embodiment of an amplifier module comprises a transmissionfiber, a dispersion compensating fiber, and at least a first pumpsource. The transmission fiber can have a relative dispersion slope. Therelative dispersion slope can be the dispersion slope divided by thedispersion. The transmission fiber can be configured to transmit asignal. The signal can be in the wavelength range of 1400 to 1650 nm.The dispersion compensating fiber can have a relative dispersion slope.The relative dispersion slope can be the dispersion slope divided by thedispersion. The dispersion compensating fiber can be coupled to thetransmission fiber. The dispersion compensating fiber can have a sign ofdispersion that can be opposite a sign of dispersion of the transmissionfiber. At least a portion of the dispersion compensating fiber can havea negative sign of dispersion and an absolute magnitude of dispersion ofat least 50 ps/nm-km for a majority of wavelengths in the signal, or forat least a portion of wavelengths in the signal. A difference betweenthe relative dispersion slopes of the transmission fiber and thedispersion compensating fiber can be no greater than 0.0032/nm over, forexample, at least a portion of a signal wavelength range, such as overat least 20 mn of a signal wavelength range, or over at least 50 nm of asignal wavelength range. The difference between the relative dispersionslopes can be no greater than 0.001/nm over, for example, at least aportion of a signal wavelength range. At least a first pump source canbe coupled to the transmission fiber. At least a first pump source canproduce a first pump beam that creates Raman gain in the dispersioncompensating fiber. The amplifier module can further comprise a firstintermediate fiber. The first intermediate fiber can couple thedispersion compensating fiber with the transmission fiber. The firstintermediate fiber can have a mode field diameter that can be less thana mode field diameter of the transmission fiber and greater than a modefield diameter of the dispersion compensating fiber. The amplifiermodule can provide net gain to a signal in the transmission fiber of atleast 5 dB, or of at least 10 dB. The amplifier module can havesufficient gain to compensate for signal losses in the transmissionline.

[0114] One embodiment of an amplifier module comprises a transmissionfiber, a dispersion compensating fiber, and at least a first pumpsource. The transmission fiber can be configured to transmit a signal.The signal can be in the wavelength range of 1400 to 1650 nm. Thedispersion compensating fiber can be coupled to the transmission fiber.The dispersion compensating fiber can have a dispersion sign that can beopposite a sign of the transmission fiber. At least a portion of thedispersion compensating fiber can have a negative sign of dispersion andan absolute magnitude of dispersion of at least 50 ps/nm-km for amajority of wavelengths in the signal, or for at least a portion of thewavelengths in the signal. At least the first pump source can be coupledto the transmission fiber. At least the first pump source can produce adepolarized first pump beam that creates Raman gain in the dispersioncompensating fiber. The first pump source can include at least twopolarized pump sources that can be polarization multiplexed. The firstpump source can include a polarization scrambler. The first pump sourcecan include a non-polarized pump source. The non-polarized pump sourcecan be a fiber laser. The amplifier module can further comprise a firstintermediate fiber. The first intermediate fiber can couple thedispersion compensating fiber with the transmission fiber. The firstintermediate fiber can have a mode field diameter that can be less thana mode field diameter of the transmission fiber and greater than a modefield diameter of the dispersion compensating fiber. The amplifiermodule provides net gain to a signal in the transmission fiber of atleast 5 dB, or of at least 10 dB. The amplifier module can havesufficient gain to compensate for signal losses in the transmissionline.

[0115] One embodiment of an optical fiber communication system comprisesa transmitter, a receiver, a transmission fiber, a dispersioncompensating fiber, a first intermediate fiber, and at least a firstpump source. The transmission fiber can be coupled to the transmitterand receiver. The transmission fiber can have chromatic dispersion at asystem wavelength. The dispersion compensating fiber can have at least aportion with a negative sign of dispersion and an absolute magnitude ofdispersion of at least 50 ps/nm-km. The first intermediate fiber cancouple the dispersion compensating fiber with the transmission fiber.The first intermediate fiber can have a mode field diameter that can beless than a mode field diameter of the transmission fiber and greaterthan a mode field diameter of the dispersion compensating fiber. Atleast a first pump source can be coupled to the transmission fiber. Atleast a first pump source can produce a first pump beam that can createRaman gain in the dispersion compensating fiber.

[0116] It is understood that various other modifications will be readilyapparent to those skilled in the art without departing from the scopeand spirit of the invention. Accordingly, it is not intended that thescope of the claims appended hereto be limited to the description setforth herein, but rather that the claims be construed as encompassingall the features of the patentable novelty that reside in the presentinvention, including all features that would be treated as equivalentsthereof by those skilled in the art to which this invention pertains.

What is claimed is:
 1. A broadband nonlinear polarization amplifier,comprising: an input port for inputting an optical signal having awavelength λ; a distributed gain medium for receiving the optical signaland amplifying the optical signal through nonlinear polarization, thedistributed gain medium having zero-dispersion at wavelength λ₀, whereina magnitude of dispersion at the wavelength λ is less than 50 ps/nm-km;one or more semiconductor lasers operated at wavelengths λ_(p) forgenerating a pump light to pump the distributed gain medium; and anoutput port for outputting the amplified optical signal.
 2. Thebroadband nonlinear polarization amplifier of claim 1, wherein a sign ofdispersion at the wavelength λ is negative.
 3. The broadband nonlinearpolarization amplifier of claim 1, wherein the magnitude of dispersionat the wavelength λ is less than 40 ps/nm-km.
 4. The broadband nonlinearpolarization amplifier of claim 1, wherein the magnitude of dispersionat the wavelength λ is less than 30 ps/nm-km.
 5. The broadband nonlinearpolarization amplifier of claim 1, wherein the magnitude of dispersionat the wavelength λ is less than 20 ps/nm-km.
 6. The broadband nonlinearpolarization amplifier of claim 1, wherein the distributed gain mediumis an optical fiber.
 7. The broadband nonlinear polarization amplifierof claim 1, wherein the distributed gain medium is a dispersioncompensating optical fiber.
 8. The broadband nonlinear polarizationamplifier of claim 1, wherein at least a portion of gain produced by thedistributed gain medium is Raman gain.
 9. The broadband nonlinearpolarization amplifier of claim 1, wherein the distributed gain mediumhas a nonlinear coefficient greater than 2 W⁻¹km⁻¹.
 10. The broadbandnonlinear polarization amplifier of claim 1, wherein the distributedgain medium has a nonlinear coefficient greater than 3 W⁻¹km⁻¹.
 11. Thebroadband nonlinear polarization amplifier of claim 1, wherein thedistributed gain medium has a zero dispersion wavelength in the range of1300 to 1800 nm.
 12. The broadband nonlinear polarization amplifier ofclaim 1, wherein the distributed gain medium has a loss at thewavelength λ of less than 2 dB/km.
 13. The broadband nonlinearpolarization amplifier of claim 1, wherein the distributed gain mediumhas a loss at the wavelength λ of less than 1 dB/km.
 14. The broadbandnonlinear polarization amplifier of claim 1, wherein the distributedgain medium is a dispersion managed gain medium with a plurality offiber segments each having a zero dispersion wavelength in the range of1300 to 1800 nm.
 15. The broadband nonlinear polarization amplifier ofclaim 1, wherein the distributed gain medium has a cut-off wavelengthshorter than the wavelengths λ_(p).
 16. The broadband nonlinearpolarization amplifier of claim 1, wherein the wavelength λ is between1400 nm and 1650 nm.
 17. The broadband nonlinear polarization amplifierof claim 1, wherein the input port is a WDM coupler.
 18. The broadbandnonlinear polarization amplifier of claim 1, wherein the output port isa WDM coupler.
 19. A broadband fiber transmission system, comprising: atransmission line having at least one zero dispersion wavelength λ_(o)and transmitting an optical signal of a wavelength λ, the transmissionline including a Raman amplifier that amplifies the optical signalthrough Raman gain, at least a portion of the transmission line having amagnitude of dispersion at the wavelength λ less than 50 ps/nm-km; andone or more semiconductor lasers operated at wavelengths λ_(p) forgenerating a pump light to pump the Raman amplifier, wherein thewavelength λ is close to at least one zero dispersion wavelength λ₀ andat least one zero dispersion wavelength λ₀ is less than 1540 nm orgreater than 1560 nm.
 20. The broadband fiber transmission system ofclaim 19, wherein a sign of dispersion at the wavelength λ is negative.21. The broadband fiber transmission system of claim 19, wherein theRaman amplifier is a distributed Raman amplifier.
 22. The broadbandfiber transmission system of claim 19, wherein the magnitude ofdispersion at the wavelength λ is less than 40 ps/nm-km.
 23. Thebroadband fiber transmission system of claim 19, wherein the magnitudeof dispersion at the wavelength λ is less than 30 ps/nm-km.
 24. Thebroadband fiber transmission system of claim 19, wherein the magnitudeof dispersion at the wavelength λ is less than 20 ps/nm-km.
 25. Thebroadband fiber transmission system of claim 19, wherein the wavelengthλ is within 30 nm of at least one zero dispersion wavelength λ₀.
 26. Thebroadband fiber transmission system of claim 19, wherein the wavelengthλ is within 20 nm of at least one zero dispersion wavelength λ₀.
 27. Thebroadband fiber transmission system of claim 19, wherein the opticalsignal has a wavelength λ in the range of 1400 nm to 1530 nm.
 28. Thebroadband fiber transmission system of claim 19, wherein the opticalsignal has a wavelength λ is in the range of 1530 nm to 1650 nm.
 29. Thebroadband fiber transmission system of claim 19, wherein a signalwavelength at λ is sufficiently low in power to avoid at least one fibernon-linearity effect.
 30. The broadband fiber transmission system ofclaim 19, wherein the Raman amplifier has sufficient gain to compensatefor losses in the transmission line.
 31. The broadband fibertransmission system of claim 29, wherein the at least one fibernon-linearity effect is four-wave mixing.
 32. The broadband fibertransmission system of claim 29, wherein the at least one fibernon-linearity effect is modulation instability.
 33. A broadband fibertransmission system, comprising: a transmission line having at least onezero dispersion wavelength λ_(o) and transmitting an optical signal of awavelength λ, the transmission line including a Raman amplifier and adiscrete optical amplifier that amplify the optical signal, at least aportion of the transmission line having a magnitude of dispersion at thewavelength λ less than 50 ps/nm-km; and one or more semiconductor lasersoperated at wavelengths λ_(p) for generating a pump light to pump theamplifiers, wherein the wavelength λ is close to at least one zerodispersion wavelength λ₀ and at least one zero dispersion wavelength λ₀is less than 1540 nm or greater than 1560 nm.
 34. The broadband fibertransmission system of claim 33, wherein a sign of dispersion at thewavelength λ is negative.
 35. The broadband fiber transmission system ofclaim 33, wherein the Raman amplifier is a distributed Raman amplifier.36. The broadband fiber transmission system of claim 33, wherein themagnitude of dispersion at the wavelength λ is less than 40 ps/nm-km.37. The broadband fiber transmission system of claim 33, wherein themagnitude of dispersion at the wavelength λ is less than 30 ps/nm-km.38. The broadband fiber transmission system of claim 33, wherein themagnitude of dispersion at the wavelength λ is less than 20 ps/nm-km.39. The broadband fiber transmission system of claim 33, wherein thewavelength λ is within 30 nm of at least one zero dispersion wavelengthλ₀.
 40. The broadband fiber transmission system of claim 33, wherein thewavelength λ is within 20 nm of at least one zero dispersion wavelengthλ₀.
 41. The broadband fiber transmission system of claim 33, wherein theoptical signal has the wavelength λ in the range of 1400 nm to 1530 nm.42. The broadband fiber transmission system of claim 33 wherein theoptical signal has the wavelength λ is in the range of 1530 nm to 1650nm.
 43. The broadband fiber transmission system of claim 33, wherein thediscrete optical amplifier is a rare earth doped amplifier.
 44. Thebroadband fiber transmission system of claim 33, wherein the discreteoptical amplifier is an erbium doped fiber amplifier.
 45. The broadbandfiber transmission system of claim 33, wherein the discrete opticalamplifier is a Raman amplifier.
 46. The broadband fiber transmissionsystem of claim 33, wherein the discrete optical amplifier is a thuliumdoped fiber amplifier.
 47. The broadband fiber transmission system ofclaim 33, wherein a signal wavelength at λ is sufficiently low in powerto avoid at least one fiber non-linearity effect.
 48. The broadbandfiber transmission system of claim 33, wherein the amplifiers havesufficient gain to compensate for losses in the transmission line. 49.The broadband fiber transmission system of claim 47, wherein the atleast one fiber non-linearity effect is four-wave mixing.
 50. Thebroadband fiber transmission system of claim 47, wherein the at leastone fiber non-linearity effect is modulation instability.
 51. Anamplifier module, comprising: a transmission fiber configured totransmit a signal; a dispersion compensating fiber with at least aportion with a negative sign of dispersion and an absolute magnitude ofdispersion of at least 50 ps/nm-km; a first intermediate fiber thatcouples the dispersion compensating fiber with the transmission fiber,the first intermediate fiber having a mode field diameter that is lessthan a mode field diameter of the transmission fiber and greater than amode field diameter of the dispersion compensating fiber; and at least afirst pump source coupled to the transmission fiber producing a firstpump beam that creates Raman gain in the dispersion compensating fiber.52. The module of claim 51, wherein the first intermediate fiber couplesthe dispersion compensating fiber with the transmission fiber with nomore than 1 dB of loss over at least a portion of a wavelength range ofthe first pump source.
 53. The module of claim 51, wherein the firstintermediate fiber is integrally formed with the transmission fiber andhas a varying mode field diameter.
 54. The module of claim 51, whereinthe first intermediate fiber is integrally formed with the dispersioncompensating fiber and has a varying mode field diameter.
 55. The moduleof claim 51, wherein the first intermediate fiber is non-integrallyformed with at least one of the transmission fiber and the dispersioncompensating fiber.
 56. The module of claim 51, wherein the firstintermediate fiber is non-integrally formed with the transmission fiberand the dispersion compensating fiber.
 57. The module of claim 51,wherein the transmission fiber is coupled to a second pump source thatproduces a second pump beam, wherein the first and second pump beams areseparated by at least 15 nm of wavelength.
 58. The module of claim 51,wherein the transmission fiber is coupled to a second pump source thatproduces a second pump beam, wherein the first and second pump beams areseparated by at least 20 nm of wavelength.
 59. The module of claim 51,wherein the amplifier module provides net gain to a signal in thetransmission fiber of at least 5 dB.
 60. The module of claim 51, whereinthe amplifier module provides net gain to a signal in the transmissionfiber of at least 10 dB.
 61. The module of claim 51, wherein theamplifier module has sufficient gain to compensate for signal losses inthe transmission line.
 62. The module of claim 51, wherein the signal isin the wavelength range of 1400 to 1650 nm.
 63. The module of claim 51,wherein the first intermediate fiber has substantially tapered geometricconfigurations.
 64. The module of claim 51, wherein at least a portionof the dispersion compensating fiber has a negative sign of dispersionand an absolute magnitude of dispersion of at least 50 ps/nm-km for amajority of wavelengths in the signal.
 65. The module of claim 51,wherein at least a portion of the dispersion compensating fiber has anegative sign of dispersion and an absolute magnitude of dispersion lessthan 50 ps/nm-km for at least a portion of the wavelengths in thesignal.
 66. The module of claim 51, wherein the signal includes multiplewavelengths.
 67. The module of claim 51, wherein the first pump sourceincludes at least one semiconductor diode laser.
 68. The module of claim51, wherein the dispersion compensating fiber includes at least twodispersion compensating fibers.
 69. The module of claim 51, furthercomprising: a wavelength selective coupler coupled to an output of thedispersion compensating fiber that provides for counter-directionalpumping.
 70. An amplifier module, comprising: a transmission fiberhaving a relative dispersion slope and configured to transmit a signal;a dispersion compensating fiber having a relative dispersion slope andcoupled to the transmission fiber, wherein a difference between therelative dispersion slopes of the transmission fiber and the dispersioncompensating fiber is no greater than 0.0032/nm over at least a portionof a signal wavelength range; and at least a first pump source coupledto the transmission fiber producing a first pump beam that creates Ramangain in the dispersion compensating fiber.
 71. The module of claim 70,wherein the difference between the relative dispersion slopes is nogreater than 0.0032/nm over at least 20 nm of a signal wavelength range.72. The module of claim 70, wherein the difference between the relativedispersion slopes is no greater than 0.0032/nm over at least 50 nm of asignal wavelength range.
 73. The module of claim 70, wherein thedifference between the relative dispersion slopes is no greater than0.001/nm over at least a portion of a signal wavelength range.
 74. Themodule of claim 70, wherein the relative dispersion slope is thedispersion slope divided by the dispersion.
 75. The module of claim 70,wherein the dispersion compensating fiber has a sign of dispersion thatis opposite a sign of dispersion of the transmission fiber.
 76. Themodule of claim 70, wherein at least a portion of the dispersioncompensating fiber has a negative sign of dispersion and an absolutemagnitude of dispersion of at least 50 ps/nm-km for a majority ofwavelengths in the signal.
 77. The module of claim 70, wherein at leasta portion of the dispersion compensating fiber has a negative sign ofdispersion and an absolute magnitude of dispersion less than 50 ps/nm-kmfor at least a portion of wavelengths in the signal.
 78. The module ofclaim 70, further comprising: a first intermediate fiber that couplesthe dispersion compensating fiber with the transmission fiber, the firstintermediate fiber having a mode field diameter that is less than a modefield diameter of the transmission fiber and greater than a mode fielddiameter of the dispersion compensating fiber.
 79. The module of claim70, wherein the amplifier module provides net gain to a signal in thetransmission fiber of at least 5 dB.
 80. The module of claim 70, whereinthe amplifier module provides net gain to a signal in the transmissionfiber of at least 10 dB.
 81. The module of claim 70, wherein theamplifier module has sufficient gain to compensate for signal losses inthe transmission line.
 82. The module of claim 70, wherein the signal isin the wavelength range of 1400 to 1650 nm.
 83. An amplifier module,comprising: a transmission fiber configured to transmit a signal; adispersion compensating fiber coupled to the transmission fiber; and atleast a first pump source coupled to the transmission fiber producing adepolarized first pump beam that creates Raman gain in the dispersioncompensating fiber.
 84. The module of claim 83, wherein the first pumpsource includes at least two polarized pump sources that arepolarization multiplexed.
 85. The module of claim 83, wherein the firstpump source includes a polarization scrambler.
 86. The module of claim83, wherein the first pump source includes a non-polarized pump source.87. The module of claim 86, wherein the non-polarized pump source is afiber laser.
 88. The module of claim 83, wherein the dispersioncompensating fiber has a dispersion sign that is opposite a sign of thetransmission fiber.
 89. The module of claim 83, wherein at least aportion of the dispersion compensating fiber has a negative sign ofdispersion and an absolute magnitude of dispersion of at least 50ps/nm-km for a majority of wavelengths in the signal.
 90. The module ofclaim 83, wherein at least a portion of the dispersion compensatingfiber has a negative sign of dispersion and an absolute magnitude ofdispersion less than 50 ps/nm-km for at least a portion of thewavelengths in the signal.
 91. The module of claim 83, furthercomprising: a first intermediate fiber that couples the dispersioncompensating fiber with the transmission fiber, the first intermediatefiber having a mode field diameter that is less than a mode fielddiameter of the transmission fiber and greater than a mode fielddiameter of the dispersion compensating fiber.
 92. The module of claim83, wherein the amplifier module provides net gain to a signal in thetransmission fiber of at least 5 dB.
 93. The module of claim 83, whereinthe amplifier module provides net gain to a signal in the transmissionfiber of at least 10 dB.
 94. The module of claim 83, wherein theamplifier module has sufficient gain to compensate for signal losses inthe transmission line.
 95. The module of claim 83, wherein the signal isin the wavelength range of 1400 to 1650 nm.
 96. An optical fibercommunication system, comprising: a transmitter; a receiver; atransmission fiber coupled to the transmitter and receiver, thetransmission fiber having chromatic dispersion at a system wavelength; adispersion compensating fiber with at least a portion with a negativesign of dispersion and an absolute magnitude of dispersion of at least50 ps/nm-km; a first intermediate fiber that couples the dispersioncompensating fiber with the transmission fiber, the first intermediatefiber having a mode field diameter that is less than a mode fielddiameter of the transmission fiber and greater than a mode fielddiameter of the dispersion compensating fiber; and at least a first pumpsource coupled to the transmission fiber producing a first pump beamthat creates Raman gain in the dispersion compensating fiber.